The best-of-seven series is tied at 2-2, with Game 5 slated for Sunday at 8 p.m. ET at Consol Energy Center here. Momentum has drastically changed numerous times, and it truly is up for grabs. And the nastiness has increased.

"We're getting into the middle of the series and you get a little more of a dislike for the other side," said Penguins veteran forward Matt Cullen.

Both teams have questions and issues, so here are the mysteries Penguins coach Mike Sullivan and Lightning's bench boss Jon Cooper are dealing with.

Fleury or Murray? The debate has been raging the entire postseason over whether or not Sullivan will replace rookie goaltender Matt Murray with veteran Marc-Andre Fleury. Murray, 21, has been solid and deserves to be in net. He was pulled in Game 4 after the Lightning gained a 4-0 lead after two periods. Sullivan made the change in order to give Murray a bit of a rest, but also to give Fleury some game action since he hadn't played since March 31, when he suffered a concussion. Sullivan said he will make a decision on Sunday as to which goalie will start Game 5.

"Marc's been a big part of this team all year," Sullivan said. "He's really helped this team get to the position that it's at with the way he's played all season. The timing of his injury down the stretch was unfortunate. It's not a perfect circumstance, but that's the nature of the business we're in, and we just try to make the best decisions and make the most of the situation we're in."

It's a strong possibility, however, that Sullivan will stick with Murray.

"We feel like we have real quality goaltenders," Sullivan said. "We've got three guys that have helped us win all year. Marc's a really good goalie. Matt's played extremely well for us, making timely saves, and he did it again [in Game 4]. So, it's a nice problem to have when you have that quality of goaltenders to choose from."

Cooper has a similar problem. Backup Andrei Vasilevskiy has played well in place of the injured Ben Bishop, who suffered an injury to his lower left leg during Game 1 and has not been cleared to return. Bishop is listed as day-to-day and hopes to be back soon, but with the way Vasilevskiy has played, it'll be interesting to see what Cooper does after Bishop is ready.

Basically, if you have a Vezina Trophy finalist ready, you don't have him on the bench. Right?

How will injuries influence the outcome? All four remaining teams in the Stanley Cup playoffs are experiencing aches and pains. That comes with the territory at this time of the season. In Game 4, the Penguins lost defensemanTrevor Daley with a left-leg injury and was seen leaving Tampa's Amalie Arena on crutches after the game. Sullivan said Daley -- who was obtained during the season from the Chicago Blackhawks -- would be re-evaluated when the team returned to Pittsburgh, but there was no further update.

"He's a tough guy to replace," Sullivan said. "He's a real good player. He's really helped change our team since he's come to join our team. He's a mobile guy. He has great offensive instincts. He helps us get out of our end zone. He's a very good player. He's not an easy guy to replace, but we've dealt with that all year.

"We've had a lot of guys go down, key guys for long stretches of time, and what I love about this group is their resilience that they find ways to have success when someone else steps up."

If Daley can't play, Olli Maatta, who was a healthy scratch for the past three games, will be back in the lineup.

Still, losing the veteran Daley will have an effect.

"Man, he's been so good for us," Cullen said. "When he got here, I didn't realize how good he was. I've always thought he was a really good player, playing against him, but what he's brought to this team has been awesome. He's a great teammate. He's a great leader. He skates well. He's done everything for us. Everywhere -- special teams, 5-on-5 -- he's been great for us, and all the young guys would say the same. He's been a great leader, and I can't say enough about him."

Sullivan said he has confidence in Maatta, if needed. Maatta struggled in Game 1 of this series.

For the Lightning, not only is Bishop's status in question, but captain Steven Stamkos (blood clot) is still being watched to see whether he can return to the lineup in this series.

Where is the real Geno? Until the third period of Game 4, Penguins' Evgeni Malkin had been struggling in this series. In the first two periods of Game 4, he was having trouble making the simple play, almost as if he was pressing too much. He was a completely different player in the third and was rewarded with his first goal of the series.

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"He's playing pretty well, honestly," Cullen said. "I know he wants to make an impact on every game, like last night the way he scored that goal. There's not a lot of guys that can do that with his big body and his skill set, so I expect him to continue to get better as the series goes on. He's a guy that can take over a game, and I've seen him take over series [while playing against him]. I think he's going to continue to get better and better."

Sullivan said he wants Malkin to focus on playing from zone to zone, rather than across the ice.

"He's such a difficult player to handle offensively, with his size and his speed," Sullivan said. "When he plays that north-south game and he forces defensemen to have to turn and defend him, he's really hard to handle. We believe that's when he's at his best, so that's where we're encouraging more of him moving forward."